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 Previous: ARRL In Action: What Have We Been Up To Lately? |  Next: Richard Garriott, W5KWQ, Back on Terra Firma |
REVISED Nov 3, 2008 13:40 ET
As Daylight Savings Time falls back, what would you do with that recovered hour? Sleep!?! No way! Here’s a pencil-consuming quiz about the characteristics of linear amplifiers. They’re easy to buy, but they have their own requirements for care and feeding that must be accommodated to work well and to last.
1) If an amplifier would increase your power output from 50 W to 500 W, while a new tribander would add 4 dB of forward gain over your existing dipole, with which would your signal be louder?
2) Will a fan rated at 0.5 cubic meters per minute be able to cool a tube that needs 20 cubic feet per minute?
3) The ac power in your shack is supplied by a 115 VAC circuit with a 20 amp breaker. Your new amplifier draws a maximum of 2 kVA. Will the circuit handle the load?
4) RG-8 is rated at 5 kV maximum working voltage. Can it withstand full-power operation with a load of 100 ohms?
5) An amplifier's transmit/receive relay has a 12 VDC coil with a resistance of 50 ohms. If your transceiver's keying relay contacts are rated at 100 mA, will they be able to switch the relay?
6) Combining the output from four solid-state amplifier modules is performed by a power combiner with an insertion loss of 1 dB at each input. If each module provides 200 W of power, how much is available at the output?
7) To avoid overdriving the amplifier input, your 150 W transceiver output must be cut to 50 W. How many dB of attenuation is required?
8) The rules state that harmonics must be at least 40 dB below the fundamental signal. A friend nearby informs you that your second harmonic is S-4 at her house while your fundamental is S-9. Assuming 5 dB per S-unit, are you legal?
9) Can a relay that switches in 5 msec support full-QSK (full break-in keying) at 35 wWPM?
10) DXpeditioners special - You're assigned to carry the power transformer for the amp that weighs 40 pounds. The airline to the island allows 30 kilograms of carry-on weight. If your coax weighs 4 ounces per foot, how many feet of cable can you carry on board along with the transformer?
Answers
1) The amplifier provides a gain of 10 log (500/50) = 10 dB.
2) One cubic meter equals 35.3 cubic feet, so the fan will supply 0.5 × 35.3 = 17.7 cubic feet. This fan is too small!
3) 2 kW at 115 VAC is a current of 17.4 Amps. Operating the amplifier on this circuit is okay, but just barely -- particularly if you are operating a high duty-cycle mode such as RTTY and there is little amperage left for any other equipment. You should run a new circuit to the shack for the amplifier. Check your local electrical codes, as well.
4) At 1500 W output (ognoring losses in the feed line), the rms load voltage will be the square root of power × load impedance = 387 V. The peak load voltage = 1.414 × 387 = 548 V, so RG-8 is well within its ratings.
5) The coil current is 12 V/50 ohms = 240 mA. The transceiver's relay contacts probably won't last long before failing!
6) A dissipation of 1 dB passes 79.4 percent of the input power to the output. The 200 W of power at each input is reduced to 200 × 0.794 = 159 W at the output of the combiner: 4 modules × 159 W = 635.4 W.
7) The output of the attenuator must be -10 log (50/150) = -4.77 dB below the input power. A 5 dB attenuator will be fine.
8) Your harmonic is 5 S-units × 5 dB/S-unit = 25 dB below the fundamental. You have a problem!
9) At 35 WPM, each dit is about 25 msec. This relay will do the job.
10) Thirty kilograms is equivalent to 66.2 pounds. The transformer leaves 26.2 pounds for cable. You can carry 26.2 pounds/0.25 ft/lb = 104 feet of cable (without connectors). Editor's Note: The original version of this answer had a subtraction error.
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